In the light of the very controversial Breastfeeding Mother who was posed on the cover of Time mag with her 3 1/2 yr old son at her breast, this is a very timely question. In fact, Nightline News (I think) did a recent update on her, because she will be posing (or has posed already) for the cover of another mag: Pathway to wellness I believe. She is a huge advocate for breastfeeding until the child is ready and supports the toddler aged child breastfeeding.
So, I was contemplating this and actually had thought of posing this question myself. I'm glad to see someone has already asked this. In the United States, it is so unusual for the older child to be breastfeeding that it makes people uncomfortable. But, I believe that this is between the child and mother. It isn't anyone else's business or place to judge, although this occurs continuously.
If our culture was not so caught up with the use of the breast as a sexual object it may become more natural and accepted. Other countries don't have this same hang up and realize breastfeeding for what it is-a natural part of the cycle of pregnancy.